Enhance Copilot instructions with build, test, and repository guidance

[Copilot]

The repository had minimal Copilot instructions. Enhanced .github/copilot-instructions.md to follow best practices for GitHub Copilot coding agent.

Added

• Build & Test: dotnet build usage, SDK version (9.0.306), test categories (BVT, SlowBVT, Functional), filtering with --filter
• Repository Structure: Core components (Orleans.Core, Orleans.Serialization, Orleans.Client, Orleans.Runtime), provider implementations (AWS, Azure, Redis, Cassandra), test organization
• Orleans Context: Grain-based programming model, Virtual Actor Model patterns, async/await conventions, grain interface inheritance
• Testing Details: Test location (test/), xUnit v3, NSubstitute for mocking, organizational structure

Preserved

• Existing C# 13 conventions
• Formatting rules from .editorconfig
• Nullable reference type patterns
• Testing style guidelines

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #9746

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

Microsoft Reviewers: Open in CodeFlow